Gas analyzing devices



Jan; 28, 1958 'J. E. McEvoY GAS ANALYZING DEVICES Filed Nov. 15. 1952BALANCEABLE NETWORK FIG. I

v INVENTOR. JAMES E. M2 EVOY A RNEY BALANCEABLE NETWORK SAMPLE 3 UnitedStates Patent 2,821,462 GAS ANALYZING DEVICES James E. McEvoy, EastCleveland, Ohio, assignor to Bailey Meter Company, a corporationofDelaware Application November 13', 1952, Serial No. 320,323 3 Claims.(Cl. 23-255 My invention isdirected to improvements in gas analyzingdevices. And more particularly, my invention is embodied in structurewhich. eliminates errors to which the prior art devices have beensubject when employing at detecting element which is sensitive tovariablesin the analyzed gas other than the one desired in measurement.The method and apparatus utilizing my improvement employs anelectrically heated filamentin oxidizing combustible gases and vaporsand which is electrically responsive to the heat of oxidation, or.combustion.

A recognized material limitation in the use of gas testing apparatus ofthe type with which I am concerned is that it is not adaptable inindicating the true extent of combustible gases and vapors .inmixturescontaining varying amounts of non-combustible vapors andgases suchaswater vapor, carbon dioxide and. others which have a substantiallyditferent cooling efiect on a heated filament thanis exerted by air. Inother words, adetecting filament. does not vary in temperature solelyfrom. the catalytic burning. The problem of analysis in this art wouldbe greatly simplifiednif such were the case. The filament must havedirect intimatecontactwith. all of the constituents of the sample inpromoting the catalytic combustion with the detected portion.Consequently, specific heat, thermal conductivity, in. short, all of'the influence of temperature variables of the sample comes into play byconducting heat away from the filament at a rate which varies with thecomposition of the sample. It might also be said that the resistance ofthe filament, and consequently, the unbalance of the including. network,has not been, in the prior art, proportionalsolely to the percentage ofthe detected element in the sample but to a combination of this factorand the physical properties heretofore referred to of the sample as amixture. Although the detected portion of the gas sample itselfcontributes to the extraneous and additional variation of the filamenttemperature by reasonof itsphysical properties of specific heat andthermal conductivity, I refer to the additional variation ascontributable, in a general sense, to the inert constituents, as asummary basis of reference. Thus, it is: the effect on the finalindication of catalytic gas analyzers by the variable inerts that myinvention eliminates.

It has been my personal conclusionthatthisdesired refinement of analysishas been recognized by many patents in the prior art in two generalapproaches. Obtainment. of this refinement has been by eitherestablishing a compensating, non-catalytic filament in the electricalnetwork in the cell of the analyzing filament, or a separate cell hasbeen providedto allow passage, over the compensating filament, of aheated. inert gas approximating the physical characteristics of thatpassing,

over the analyzing filament. The disadvantages of. these schemes areobvious. The compensating, filament in the single analyzing cell wouldhave 'to be coatedwith, or

made of, a substance whichvvould're'nder it'non-catalytic, withconsequent non-parallel response with measuring filament to the physicalcharacteristics'ofthe 'inerts; "or

it isv impractical to flow through the separate compensating cellv aninert which has variations in its physical characteristics parallel withthose of the medium flowing through the analyzing cell.

Anexample. of companion cell compensation structure is. to be. found inat least the disclosure of the patents to Sullivan 2,197,370 and2,404,993, while there are other patents such as Morgan et al.2,211,627, 2,204,966 and2,273,981 which disclose coating or constructingthe compensating filament of a substance which is noncatalytic. Thepatents to. Miller 2,083,521 and Sullivan 2,310,472 are of interest intheir recognition of. this problem, but.these patents do not solve theproblem as does the present apparatus.

The general observations in Jacobson 2,378,019 are of interest due tohis observationthat when a gas sample with a low concentration ofcombustibles contains a considerably diiferent amount of water vapor orcarbon dioxide from that in the air used for establishing the initialzerowsetting of analyzers using other means' of compensation the totalheat loss due substantially to thermal conduction, convectionand directtransport by the moving gas, may be very large and may be even greaterthantheheat producedby oxidation of the combustibles. Asdescribed byJacobson, an instrument of this kind has been made useable to apractical degree by removing. moisturefrom the sample and the air used,for zero. calibration through afdrying medium. such; as calciumchloride, activated alumina or the like, orthrough a water bubbler whosetemperature is kept'cqnstant by means of a thermostat; Aside from theinconvenience to the user, thesemethocls of eliminating moisture bydrying the sample are not always applicable becauseany available dryingmaterial would absorb someof the comhustiblevapors while absorbing watervapor.

Jacbson goes on to classify the prior artmethods of compensation intotwo general methods. He firstsees thatcompensator filament as totallyenclosed in a special housing or in a sealed gas-tight glass or,metaltube so .that the filament is constantly surrounded by the sameair, or. any part of the sampleis prevented. from reaching it. Actuallythis appears closely akin to. the method I have outlined in the priorart for rendering the compensating filament non-catalytic by coating it.As Jacobson points out, this method provides very goodcompen- .sation.for voltage changes but does very little in the direction ofcompensating for Water vapor, carbon dioxide or other non-combustibleconstituents in the sample, which I. term inerts.

Secondly, Jacobson described the method of equal exposure of thedetecting and compensating filaments to the sample but with theprovision of acompensating filament of. relatively large size to reduceits resistance andthereby decrease its-temperature below thesurfacecombustion temperature. In a certain sence Jacobson treats this methodas classifiable withv the'coating technique I have described. He devotessome detail to'describing the disadvantages of equal exposuretothesample under this method of compensation, but he does; not further giveclassification to the category I have'recog- :nized as. parallelexposure ofthefilaments, the detecting "am concerned with providing animprovedmeans for determining the percentage of freeoxygen and/or.gaseous combustible in a flowing gas sample, such as, for exam,

pie, a sample stream of fiue gas from a combustion process or exhaustgases from an internal combustion engine. The gases with which I amconcerned will generallycontain nitrogen, carbon dioxide, free oxygen,carbon monoxide, methane, or other unburned combustible. I desirablyascertain the percentage of free oxygen and, separately, the percentageof unburned combustible in the gas.

It has been explained in some detail in the patent to Johnson 2,420,430that to test gases for combustibles, it is often necessary to provide apredetermined quantity of oxygen to insure the burning of anycombustible in the gas sample. On the other hand, to test a gaseousmixture for its free oxygen constituent, it is necessary to pass overthe heated catalyst filament, a mixture of a uniform rate of flow of astream of the sample to be analyzed and a" uniform rate of flow of astream of a gaseous fuel for combining with the free oxygen of the gassample stream. As explained in Johnson, it, is

known that the fuel supplied to such a system may be a gas, such ashydrogen, or a vaporized liquid fuel such as methyl alcohol; Johnsondiscloses the use of vaporized liquid fuel for this purpose.

Other patents have dealt with this dual analysis of "a single gassample, or separate analysis of two gas samples, by simultaneous, orserial, catalytic burning. Simultaneous provision of sources of oxygenfor the comfbustible analysis and hydrogen for the oxygen analysiswithin the gas or gases has been contemplated in Kennedy 1,333,850 andMorgan et al. 2,273,981. Morgan et al. has even disclosed a generatorwith which to produce hydrogen. However, none of the prior art hascontemplated plenary usage of the out-put of an electrolytic generatorin a catalytic analyzer, the generator and analyzer combinationproviding simultaneous, and separate, analysis for the combustible andfree oxygen content of gas samples.

As one of the primary objects, my invention provides a gas analyzingdevice capable of simultaneous and independent analysis of a single, ortwo independent, gas

samples for free oxygen and for combustible content.

Another primary object of my invention is to provide a gas analyzerwhose record and/or indication will not reflect variations of inertconstituents in the gas sample with parallel independence from separatesources of fuels for combustion with the analyzed constituent of thesample.

Another object of my invention is to provide a gas analyzer having meansfor generating fluids which will combine with the constituents for whichthe analysis is made.

Another object of my invention is to provide a 'gas analyzer whichgenerates oxygen and hydrogen for si- 'multaneous catalytic combustionof the combustibles and invention.

Fig. 2 is an exploded view of a filament and shield used in the analyzerof Fig. 1.

.Referring to Fig. 1, I have shown therein, somewhat diagrammatically,and not to any scale, the basic units of an apparatus combinationcapable of analyzing a gas sample simultaneously for its combustibleconstituents and free oxygen. The gas sample is cleaned and prepared foranalysis by an apparatus not shown here but well known in the art. WhatI have shownis a gas sample tube 1 through a furnace wall 2 for theinitial extraction of the sample to be prepared for analysis. The

apparatus for making this extraction, as well as that for cleaning andpreparing the sample for introduction into: the analyzing apparatus, isto be visualized as incorporated at some location along passage 3.

The cleaned sample flow in passage 3 is led to the base central housingstructure 4 where the catalytic combustion takes place. This housing 4forms an area into which is flowed, not only the sample of gas to beanalyzed, but the substances to be mixed with the sample prior tocatalytic combustion. The electrical networks which are generallyresponsive to the results of the catalytic burning are shown as dualunits separately responsive to the two analyses and indicating,separately, the combustible constituent of the sample and the freeoxygen constituent. The housing structure 4 itself has a desirablefunction other than mere support of various element combinations. Toperform its function most efiiciently this housing is preferably of ametallic material which gives a'large heat storage for insulation of theresponsive elements'of the analyzer from ambient tem- Structure formaintaining the temperature'of this housing 4 at a desirable level aboveambient tmperature will be described subsequently.

Specifically, the metallic housing 4 has three different streams offluids flowing into it. It is necessary for the successful operation ofcatalytic analyzers of this type that these fluid supplies to theanalyzing cells in the housing 4 be controlled as to their flow rates.Consequently, the sample in conduit 3 is held to a constant pressure bymeans of regulator 5 prior to introduction into the housing 4.- Thisregulator 5 may be mounted directly upon hous ing 4, and it may be quitesimilar to the regulators of the patent to Johnson 2,43 8,973.

Tracing the path of the sample in conduit 3 on through the regulator 5and into the housing 4, it may be seen how the sample is split into twobranches for simultaneous introduction into two separate analyzingcells.

In'eachbranch is established an orifice for precise determination of therates of flow required for proper cata lytic action in the analyzingcells. Orifice 6 meters its portion of the sample into the cellanalyzing the gaseous sample for its combustible content. Orifice 7performs a similar'function on its portion of the gaseous sample whichwill be analyzed in the second cell for the free oxygen content.

Prior to" either sample portion being introduced into the respectiveanalyzing cell, each portion is mixed with an" element which willpromote combustion of the constituent to be detected on the detectingfilament in the analyzing cells. Considering, first, the analysis madein cell 8, theprovisions are to be noted for supplying oxygen with thesample which is metered through orifice 6. Generally, in the prior art,a source of compressed air has been disclosed for this purpose, or atank of oxygen has been controlled through various valves to mix itscontents with' the 'gas sample prior to catalytic combustion. None ofthese provisions of the prior art are necessary with my inventionbecause electrolytic generator 9, as an integrated portion of myanalyzer, forms a convenient, simple and eflicient source of oxygen formixing with the gas sample.

It has been found quite feasbile to design the size of generator 9 tosupply the maximum quantity of oxygen needed to mix'with the sampleportion analyzed in cell 8. No valves or flow controllers betweengenerator 9 and the analyzing cells are necessary as control of thebasic output of the generator is a function of the electric powerapplied toits eelctrodes. Therefore, pipes 10 and 11, conducting oxygenand hydrogen from generator section 9 into housing 4, may be quiteshort; in fact, the

generating section may be attached directly to the housing 4,considering only the connections necessary to gain access to thegenerator for maintenance and addition of water and electrolyte.

Considering the other product of the generator 9 which a portiontif thegas sample "is analyzed'for its free oxygen content "as'it was "analyzedfor its combustible content in cell 8. The gas sample is mixed withhydrogen which combines with the'free oxygen content resistance of thedetecting filament in indication of the tree oxygen content of thegaseous sample portion.

It is, of course, to be appreciated that with initial considerationgiven the size of the electrical signal needed for actuation of abalanceable electrical network, thesize of the filaments, their cellsand the rates of'flow'required for the sample portions and the-oxygenand hydrogen must be carefully considered in a unitary design. The moreflexible and'logical point of-adjustment of the various variables is inthe size of the orifices 6 and 7 for the sample of gas. Theseorificestructures may be made up as easily inserted fixtures withvarious, specified sizes of passages for the gas sample. A-setoforifice=sizes can be predetermined which will establish rangesof-actuationfor the balanceable networks to'span'the wide quantitativevariation, in the samples, of the analyzed constituents.

The output of the electrolytic generator 9 is fixed by -the size of theelectrodes, the electric power applied to them and the degree to whichthe electrodes are immersed in their'eelctrolyte. 'It is generallyanticipated that an excess of oxygen and hydrogenneededfor catalyticcombustion of theanalyzed constituents 'Wlll be supplied the cells 8'and12. Thisoutput is basically set by the size 'of the generator'sectionand the size of'conduits 1'0 and "'11 are'fixedfor themaximumoutput expected of generator 9 so excessive back-pressure will not buildupin the housing. -As the output of thegenerator will fluctuate withvariations in the voltage of the electric power applied,-aconstant-voltage transformeris considered necessary under normalconditions of operation. 'I have indicated,- diagrammatically, at 13both a transformer and a system of rectification for the A.-C. appliedto the electrodes of thegenerator.

The electrodes of the genera-tor are shown immersed 'below the level ofthe electrolyte. It is commonpractice to give 'theinnerelectrodecorrugations in order that its area will compare-with"that of the outer electrode. Both electrode 14 and "electrode 15 aresupported by sturdy metallic posts which also serve as conductorsfor thecurreritfrom 1'3.

The-combination of the generator has a particular 'fea- However,generators of the takes place to a detrimental extent. 'To retain theadvantage of the asbestos cloth in its ability to allow free passage' of'the ions in'the electrolyte between electrodes,

while eliminating the intermingling, I have provided a metallic shieldas a substitute for an upper portion of the asbestos barrier.Thearrangement of the metallic shield 17 and asbestos'shield 16 can beclearly seen in Fig. 1. The metallic shield 17 is-atta-chedto the top ofthe genera- -tor, extending down, between the electrodes, to the exipected minimum level of-the electrolyte. The asbestos shield 16iswired, or otherwisesolidly fixed, to the lower end :of the shield -17and may extend down to the bottom of the generator, certainly far enoughto prevent the gas discharging*from-theelectrodes from crossing to thewrong side of the barrier.

Thus, as the level'of'the electrolyte lowers, the metallic shield .17continues to insure ,positive separation of "the accumulated, .generated"gas prior to usageby"the analyzer section. The shield retention withinthis constant heat source.

"17 is fixed at a length 'whichw'ill allow the electrolyte usage tooccur over a substantial, predetermined period without sinking to thelower asbestos shield '16. This arrangement is unique in generators ofthis size'andin generators'having a constantdemand upon them.

It is-also to be noted, at this point, that the "location of theorifices and the various passages for the sample, oxygen and hydrogenwithin the metallic housing 4 provides-aconvenient structure formaintaining the tempera ture of the various fluids at a constant levelas they are metered. The-housing 4 not only heats the gaseous fluids toa common, consistent temperature, but maintains the orifice structuresthemselves unchanging by reason of their To maintain housing 4 at adesired temperature level, heater 18 is conveniently recessed into thehousing and is placed under control of thermostat 19 by means of relay20. And it also might be noted at this time that the temperature levelatwhich housing -4' is maintained by heater 18 insures that water formedduring the catalytic combustion will not collectin the cells and varythe temperature of the filaments by reason of its heat absorbingproperties. As amatter of fact, should now begin to strongly impress anyone versed in this art that the structure of my analyzer gives adequateprovisions for safeguarding the fact that the variation in temperaturebetween the measuring and compensating filaments of the analyzing cellswill be solely a function of the degree of catalytic combustion on themeasuring filament.

And finally, it should be obvious that if each set of two filaments incells 8 and 12 are established, essentially, as legs in a'Wheatstonebridge included in balanceable networks 21 and 22, the restoration ofbalance to these networks will give the desired indications ofcombustibles and free oxygen present in the gaseous sample extractedthrough conduit 3.

The advantages of the analyzer over the one disclosed in Johnson2,420,430 may be readily apparent from one standpoint it it is observedthat the arrangement of Johnsons Fig. 3 is not necessary with thepresent invention. As the present analysis is made in parallel, andindependently, in the present invention, the compensation of the oxygenanalysis for air added need not be made. However, it may be desirable,in some instances, to utilize a combination of the two analyses. This issimply provided for by a single balanceable network instead of 21 and 22or by the mechanical linkage I have shown as actuated by the twobalancing motors. Essentially link 23 is simultaneously positioned bythe motors of networks 21 and 22. The resultant of these positioningsdetermines the vertical position of link 24 which actuates pilot valve25 and for indicator 26. The output of valve 25 can be utilized tocontrol, desirably, from the two variables, combustibles and oxygen. Itthe single balanceable network is desired, the arrangement could besimilar to that of Fig. 13 in Johnson 2,420,430.

A final feature of this invention is disclosed in connection with theflow valve, or regulator 5, similar to that of Johnson 2,438,973. It isquite possible that regulators of this nature may be required on theother fluid supplies introduced into analyzer 4 as representative of anyconsurner, or utilizer, of fluids under pressure.

It must next be noted that regulator 5 has at .least one verticallyreciprocating part, which assumes various heights, depending on thevariations of pressure in the source of fluid supplied. If aninterruption should occur in the fluid supplied, the reciprocating part,or member 27, would drop, by gravity, to the bottom of its bore, as itis called in the Johnson patent. Normally, the fluid flows into the borebelow the conical head of the member 27 and the resulting pressure builtup in the borerraises the member 27 until a cross-slot is uncovered invarying degree to allow a flow of the fluid to atmosphere. A variableportion of the fluid is wasted. in.order.that,a constant predeterminedpressure will be maintained in A 7 the bore depending upon the weight ofthe member 27 and cross-sectional area of the conical head. I regard thewaste passage formed by the bore slot and head of member 27 asessentially a variable orifice regulating the pressure of the fluid, themember 27, consequently,

, assuming various positions within a predetermined range.

With the foregoing structure available in regulator 5, and theequivalent of member 27, may be found in other regulators, I suddenlyperceived that the motion provided could be utilized to provide a signalor control action if the fluid supply should depart from its expectedrange. However, there is only a very small amount of power available inthese members represented by 27. Actuation of a circuit switch by member27 is not a straightforward problem. Eventually I arrived at thestructure disclosed for accomplishing the required function. I attacheda small magnet 28 to the elongated end of the member 27 and arranged ametallic disc 29 on the end of actuating arm 30 of switch 31 at a pointwhere the magnet would attract the disc a sufiicient distance to actuatethe switch when the fluid sample pressure dropped to a predeterminedminimum.

It is, of course, possible that other types of regulators can employ theprinciple of my invention and other fluids than in the presentdisclosure could be so monitored. It is also possible to have a switch31 in circuits other than the simple annunciator circuit of light 32specifically disclosed. Also it is simple to arrange the magnet 28 anddisc 29 so that actuation of switch 31 will occur at the other extremeof its range. Once switch 31 is actuated, it can be included in acontrol circuit to send a control valve, or other member, to a safeextreme of its range of operation. Other results may be required of myactuating mechanism and it is flexible enough to be readily adapted tomany circumstances which can be envisioned.

Fig. 2 is ofiered as a disclosure of the essential structure of thefilament supports and novel elements associated therewith to be found inboth cell 8 and cell 12. In each cell, insulating base 40 supports theterminals and filaments 41 and 42. The filament 42 is the measuring onewhile 41 represents the comparison filament. A single shield, or shell,fits over both of these filaments to prevent direct impingement on thefilaments of the mixture introduced into cells 8 and 12. Shield-shell 43fits over the filaments and attaches snugly to its base 40.

The structure of shield-shell 43 is unique in arrangement. It is to benoted that a partition 44 isolates filaments 41 and 42 from each other.With this internal isolation of filaments provided, a fine metallic meshscreen 45 is carried across one-half of the end of shield 43 and aplate, bearing an aperture 46 therethrough, is carried across the otherhalf of the end of 43. With this struc ture of the cells complete, thesample gas does not flow over the filaments as in so many prior artdevices but is introduced along the outside of the walls of shield 43,and this tangential introduction swirls and thoroughly mixes theconstituents of the streams coming into the cells. Actually, thatportion of the mixture which reaches the filaments is accomplished bythe phenomenon of diffusion, common to gaseous fluids.

With the structure defined above, catalytic combustion on filament 41 isprevented from being identical to the catalytic combustion occurring onfilament 42. This desired result is obtained very satisfactorily bygreatly limiting the rate of diffusion to the compensating filament 41by means of the small aperture 46 in shield 43. The rate of diffusion tothe compensating filament 41 is reduced so drastically by aperture 46that insignificant catalytic combustion occurs on the surface of thisfilament while at the same time the rate of difiusion maintains apassage of the mixture over the compensating filament 41 which isproportional in rate to that over measuring filament 42. Whereas, withthis arrangement, all the physical properties of the gaseous sample andair or oxygen or hydrogen as well as the products of catalyticcombustion have a cooling effect on measuring filament 42, simultaneouswith its temperature elevation due to catalytic combustion, the sameeffect is maintained proportionately upon compensating filament 41. Withproportionality in effects on the opposed sensitive bridge elements ofnetworks 21 and 22, with the exception of the raised resistance of themeasuring filament 42, due to the catalytic combustion thereon of theanalyzed constituent, the resulting unbalance of the networks 21 nd 22truly reflect the percentage by volume of these constituents existing inthe analyzed gaseous sample.

The unitary construction and assembly of the base 40, filaments 41, 42,and shell 43, of Fig. 2 provides the possibility of pro-assembly andcalibration, for ready removal and replacement, without change in theindividual or overall characteristics of the system. Furthermore, thisunitary assembly of measuring and comparison filaments, insures anunchanging relation of heating, ambient effects, and the like.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A catalytic gas analyzing cell structure removable from andinsertable into a gas sample receiving chamber in a heated block memberincluding, a non-metallic insulating head portion supporting two similarfilaments and their terminals, the filaments supported in spacedparallelism, a metallic shell having an open end closely engaging thehead portion, surrounding the filaments, and having a central dividingpartition extending between the filaments to form two chambers, onearound one filament and the other around the other filament, and endclosure structure for the other end of said shell having gas difiusingopenings of different degree into the two chambers from said gasreceiving chamber.

2. A continuous gas analyzing apparatus, including a heated metal block,a combustion chamber in the block, a heated catalytic filament mountedin the combustion chamber, a mixing conduit in the block and connectedto the combustion chamber, a gas pressure regulator having a pressureresponsive valve member freely movable vertically relative an orificeprovided in the regulator to maintain the gas pressure on one side ofthe orifice constant, a source of gas to be analyzed, a conduitconnecting said source of gas to the regulator, another conduitconnecting the regulator and the said mixing conduit, a source of gas tobe combined with a constituent of the analyzed gas, a conduit connectingthe source of gas to be combined with a constituent in the analyzed gaswith the mixing conduit, a balanceable electric network including theheated catalytic filament, a switch actuated by the movable valve memberof the regulator at a predetermined point in the range of regulatingmovement, and an electric circuit controlled by the switch and adaptedto signal a predetermined position of said movable valve member.

3. The apparatus of claim 2 wherein the movable valve member of theregulator has a magnet at one end and is positioned for operation of theswitch.

References Cited in the file of this patent UNITED STATES PATENTS1,356,598 Cahill Oct. 26, 1920 1,529,470 Dowd Mar. 10, 1925 1,770,059Barber July 8, 1930 1,918,702 Hebler et al. July 18, 1933 1,988,841Hayward et a1 Jan. 22, 1935 2,273,981 Morgan et al Feb. 24, 19422,335,032 Sullivan Nov. 23, 1943 2,378,019 Jacobson June 12, 19452,404,993 Sullivan July 30, 1946 2,420,430 Johnson May 13, 19472,438,973 Johnson Apr. 6, 1948 2,488,758 Binford Nov. 22, 1949 2,652,315McEvoy Sept. 15, 1953

1. A CATALYTIC GAS ANALYZING CELL STRUCTURE REMOVABLE FROM ANDINSERTABLE INTO A GAS SAMPLE RECEIVING CHAMBER IN A HEATED BLOCK MEMBERINCLUDING , A NON-METALLIC INSULATING HEAD PORTION SUPPORTING TWOSIMILAR FILAMENTS AND THEIR TERMINALS, THE FILAMENTS SUPPORTED IN SPACEDPARALLELISM, A METALLIC SHELL HAVING AN OPEN END CLOSELY ENGAGING THEHEAD PORTION, SURROUNDING THE FILAMENTS, AND HAVING A CENTRAL DIVIDINGPARTITION EXTENDING BETWEEN THE FILAMENTS TO FORM TWO CHAMBERS, ONEAROUND ONE FILAMENT AND THE OTHER AROUND THE OTHER FILAMENT, AND ENDCLOSURE STRUCTURE FOR THE OTHER END OF SAID SHELL HAVING GAS DIFFUSINGOPENINGS OF DIFFERENT DEGREE INTO THE TWO CHAMBERS FROM SAID GASRECEIVING CHAMBER.